22 JET-PHYSIK: QCD-TESTS III Bestimmungen der starken KopplungWeltmittel (NNLO)S(MZ)=0.1187(20) [PDG] S(MZ)=0.1182(27) [Bethke]HERA-Mittel (NLO)S(MZ)=0.1186± (exp)±0.0050(th)In out picture of collinear factorisation of hard and soft contributions tothe Xsection we can divide the calculation in the PDFs (input from inclusive measurements) and the hard partonic matrix element which we then convolute for each order in alphas and for each quark flavour and the gluons seprately.Depending on how you look at the Xsection you can do all the things mentioned.¶ konsistente Einzelmessungen in vielen Prozessen. ¶ “Laufen” der Kopplung wird von QCD beschrieben. ¶ HERA-Messungen: vergleichbare Genauigkeit.Bonn, 11. Mai 2006TSS: QCD bei HERA

36 JET PHYSICS: UNDERLYING EVENT?In out picture of collinear factorisation of hard and soft contributions tothe Xsection we can divide the calculation in the PDFs (input from inclusive measurements) and the hard partonic matrix element which we then convolute for each order in alphas and for each quark flavour and the gluons seprately.Depending on how you look at the Xsection you can do all the things mentioned.Bonn, 11. Mai 2006TSS: QCD bei HERA

42 EXTRACTION OF S Typically done using interpolationInput:Dependence of cross-section on s(MZ) in each bin from NLO pQCD with different input s(MZ) values. for example with MRST or CTEQ4 (3/5 different s values, to 0.122).Parame- trisation:Functional dependence on s(MZ) then approximated by -- Ai, Bi determined in fit.Result:Use function to map measured cross-section to value of s(MZ).Finally combine single points or evolve to correct scale to show running of alphas.Plot is stolen from H1.Bonn, 11. Mai 2006TSS: QCD bei HERA

44 SUMMARY ON HERA S (C. Clasman)Consistency of all results is an important check!Nice demon- strating of running in HERA data.Bethke with world average includes only NNLO results (tau decay etc.), HEA only NLO  l;arger theo error.One should mention that more and more combined effort between H1 and ZEUS (alphas, F2, fits).HERA : S(MZ)=0.1186±0.0011(exp.)±0.0050(th.) (only NLO) Bethke: S(MZ)=0.1182± (all NNLO)Bonn, 11. Mai 2006TSS: QCD bei HERA

45 ACCESS TO PDFs WITH JETS Hope: Improve gx(x,Q2) at high values of xProblemGluon density xg(x) at high x basically unknown – constrained only by momentum sum rules and Tevatron jets with large ET.Uncertainties are very large: – 15% at x=0.3, – 200% at x=0.5.IdeaJet data provide access to high x gluon via BGF process.sProblems… exploiting this feature: … Many …CTEQ is only simple parametrisation, no real fit – but they make a secret from it.QCDCBGFBonn, 11. Mai 2006TSS: QCD bei HERA

47 METHOD How to get jets NLO in <1s?Problem• Evaluation of NLO jet cross-sections: 8 hours for 50M events. • PDF fit requires O(1000) evaluations  PROBLEM!Assumption• PDFs are approx. flat in small bins of x and f.Flat!• Divide phase-space in small x-f bins. • Remove `constant’ PDF bit from integration in each bin, • integrate the in each bin for once and for good and • store the integrated values in ASCII table.Grids become potentially very large, especially when complicated interpolations are triedWhich migt be needed at large values of x where steep PDF behaviour …Table!Simplification– from integration of PDF and hard scattering matrix element – to multiplication of constant PDF and tabulated and summation over all bins of x and f.  0.01s for NLO !!!!!Bonn, 11. Mai 2006TSS: QCD bei HERA

48 THE METHOD WORKS for DIS, photoproduction, heavy flavours, Tevatron …Comparison of normal NLO calculation with NLO cross-sections calculated with the formulae on the slide before.ZEUS inclusive jets.D0 inclusive jets.Actually now done also by Carli for HERA-LHC – with similar results.Bonn, 11. Mai 2006TSS: QCD bei HERA

49 INPUT DATA in addition to F2Photo- production dijetsAnd this with only the limited stats from 9697! But stat not entscheidend!High-Q2 inclusive jetsBonn, 11. Mai 2006TSS: QCD bei HERA

50 EFFECT ON STRONG COUPLING Very much constrained by jets dataStrong correlation between s and the gluon density for in- clusive F2 data  large increase in gluon uncertainty when s free!F2 + jetsF2 + aloneWhen including jets the gluon error does not blow up so much when fitting alphas and gluon together.Jet cross sections directly sensitive to s via *g  qqbar (coupled to gluon) and via *q  qg (NOT coupled to gluon)  s NOT as strongly correlated to gluonBonn, 11. Mai 2006TSS: QCD bei HERA

60 EVENT SHAPES: MEANS as a means to test power corrections (in high Q2>80 GeV2)IdeaAlternative way to describe non- pert. hadronisation corrections to event shapes. PC depend on one universal parameter 0. Example: Thrust (long. collimation)Idea from webber et al. So no need to do LO mC hadronisation corrections.Why not for jets etc.? Not inclusive enough an object?P is …????Sum over all particles in current of Breit (except for y2 and kout, there all breit).PC approach works for means …… but fit to S and 0 difficult.Bonn, 11. Mai 2006TSS: QCD bei HERA

62 CONCLUSION AND OUTLOOK HERA is a great machine for QCD!HERA ZEUS– Aim of lumi upgrade reached: 5-times higher lumi than HERA I– Hoping to get 700pb-1 on tape before shutdown in 2007.– Experiments and machine getting a bit old … need lot of caretaking.HERA– HERA is an ideal laboratory which allows to adjust the parameters with which one wants to investigate QCD.– HERA contributing significantly to - tests of pQCD (factorisation, PDF universality). - determinations of QCD parameters (PDFs, S) tests of theories / models (PC, BFKL, …)Future– More statistics will allow for higher precision and thus better insight into QCD in few places. .– Theoretical progress might improve our understanding of some problematic aspects of QCD– Hopefully more insight into difficult subjects like parton evolution, DIS-photoproduction transition, … . And input for LHC!Laboratory because you can adjust the experimental conditions rather well.Bonn, 11. Mai 2006TSS: QCD bei HERA

65 ENERGY FLOW OBJECTS IN ZEUS Combining tracks and calorimeter informationIdea• Make best use of both detectors calorimeter at high momenta, tracker at low momenta.Method• Combine cells to ‘cone islands’. • Match tracks to islands. • Decide which information to use.I think in H1 this used to be called CLIM or FSCOMB objects – any changes since then?Right top plot: energy/momentum resolution as function of particle E/pT. Clearly visible:Point where change from tracker to CAL is sensible.Bottom plot: left: Resolution for calorimeter (hadronic) E-pz as function of yjb. Result: EFOS (solid line best). Top line is cells alone.Bottom right: relative Error on hadronic E-pz: 20% improvement on yjb at low values.Result• Resolution in pT,h and (E-pz)h. improves by 20%.Bonn, 11. Mai 2006TSS: QCD bei HERA

68 INCLUSIVE JET CROSS-SECTIONS (DIS) 1998-2000 versus 1996/97– As expected increase from /97 to at high ET and Q2.– Change described by NLO  everything consistent.96/97 was about 36 inverse pb.Interest to measure in new data although no big improvement in precision expected – we are not dominated by theory.Bonn, 11. Mai 2006TSS: QCD bei HERA

70 TRI- AND DIJETS IN DIS in the same phase-space.Idea:In ratio of dijets to trijets some uncertainties cancel out (lumi, partly energy scales, etc.)Theory:Nice possibility to extract the strong coupling with rather small uncertainties!Bonn, 11. Mai 2006TSS: QCD bei HERA

75 EFFECT ON STRONG COUPLING Very much constrained by jets dataMassive improvement on value and error. Also correlation a bit decoupled because with QCDC no correlation In common fit of alphas and gluon gluon error does not blow up so much if jets are included.  next slideBonn, 11. Mai 2006TSS: QCD bei HERA

76 PHOTON PDF FROM DIJETS … just to point out the problems …Low ET jets (6 GeV) are sensitive to the gluon content in the photon.Large contributions from underlying events  subtraction procedure.Size of correction depends on model!  large uncertainties!Difference between PYTHIA and PHOJET from regularisation procedure for the behaviour of soft particles  different behaviour  large uncertainty for softest jets large uncertainty for gamma PDF extraction.Quarks in gamma well restricted from LEP, gluon more interesting.The plots old stuff: 7.2 pb^-1 from 96. Jets abouve 4 and 6 GeV, Mjj > 12.Gluon density in the photon as extracted from H1 dijet p data.Agreement with other determinations; very large errors.Bonn, 11. Mai 2006TSS: QCD bei HERA